This invention pertains to adaptive hybrids which serve to, for example, eliminate or reduce "talker" echoes on telephone channels.
Prior art adaptive hybrids and their uses are described, for example, in Messerschmitt, David G., "An Electronic Hybrid for Adaptive Balance In Telephony," IEEE Transactions on Comm., August 1980, pp. 1399; Dotter, et al., "Implementation of an Adaptive Balancing Hybrid," COM-28, p. 1408, August 1980; and White, Stanley A., "An Adaptive Electronic Hybrid Transformer," IEEE Transactions on Comm.," December 1972. However, the previous schemes have two major shortcomings:
1. They are not sufficiently versatile and effective against the widely differing echo path characteristics encountered on telephone channels.
2. The adaptive algorithms, in general, tend to be costly to implement on a per-stage basis; and the effectiveness is usually limited by using only one or two adaptive stages.
FIG. 1 shows a block diagram of a widely known prior art adaptive hybrid 100. Adaptive hybrid 100 is trained on the signal from the local transmitter received via port 103. This is usually done while the signal, normally received from the far end, is absent.
Filters 101 and 102 approximate the characteristics of nominal short and nominal long "near-end" echo paths, respectively. Interpolator 104 is automatically adjusted in response to the operation of adaptive algorithm 105 in an attempt to provide an echo estimate to summer 106 that closely approximates the echo. Circuit 106 subtracts the estimated echo from one output of the fixed hybrid, and this output normally includes echo plus received signal.
A major limitation of the prior art adaptive hybrid 100 of FIG. 1 arises from the fact that a substantial percentage of echo paths (even near-end echo paths) differ widely from any characteristic that is capable of being provided as an interpolation between the two filters 101 and 102. This fact is indicated by the scatter diagrams in Gresh, P. A. "Physical and Transmission Characteristics of the Customer Loop Plant" BSTJ, Vol. 48, pp. 3337-3385, December 1969; and Manhire, L. M. "Physical and Transmission Characteristics of the Customer Loop Plan," BSTJ, Vol. 57, pp 33-59, January 1978. Therefore, the prior art approach of FIG. 1 provides little, if any, attenuation of some echoes.